My buddy did a Free Air setup in his gen 3 (now wrecked)
If you dont know, that is when you mount some type of baffle board where the back seats are (you can build to fit behind seat or take up where the back seat was) I think he used 1 1/2" high density fiberboard, carpeted it, modified and cut the pass through seatback/trunk opening to accomadate 2 12s and 2 tweets, sealed up the entire trunk with sound deadening material (like dynamat) you are now using your entire trunk as a speakerbox.
It should be noted though that only certain subs are suitable for this type of application, (check mfg. specs) for example, Pioneer makes a free air sub which handles a little more power and is less sensitive w/ a slightly differnt basket design. 2 of those in his cam pounded hard.